1. Field of the Invention
The invention relates to a method for measuring the in pressure sensitive adhesive, and more particularly to a method for measuring the degree of crosslinking in pressure sensitive adhesive, having the advantages of simplification and short measurment time.
2. Description of the Related Art
Pressure-sensitive adhesive refers to an adhesive which bonds to an application surface as a result of applied pressure. In general, the pressure-sensitive adhesive comprising organic polymers exhibits adhesion over a long period of time. Since the pressure-sensitive adhesive has high cohesive strength and superior elasticity, the pressure-sensitive adhesive is likely to be bonded to the surface of various materials under gentle finger pressure and be adhered and removed without residue. Accordingly, the pressure-sensitive adhesive composition must comprise components having high elasticity and adhesion respectively.
Due to the convenience, pressure-sensitive adhesives are widely used in the fabricating process of liquid crystal displays, for example, serving as the adhesive layer of polarizing film.
A polarizing film, also called a polarizer, is utilized to control the polarizing direction of the incident light and allow the polarized light to pass through. Thus the polarizer can be applied in a LCD display panel to increase the contrast value. Referring to FIG. 1, a conventional polarizing film 10 comprises a PVA polarizing base film 12. The method of preparing the PVA polarizing base film 12 comprises dipping a stretched polyvinayl alcohol film in a dichromic dye and baking the obtained film. First and second triacetyl cellulose (TAC) films 14 and 16 are respectively formed on a first and second sides of the PVA polarizing base film 12. To prevent the PVA polarizing base film 12 from scratching, a protective layer 18 is formed on the first triacetyl cellulose (TAC) films 14. A pressure-sensitive adhesive layer 20 is formed on the second triacetyl cellulose (TAC) films 16 and provisionally mounted on a release film 22 before being pasted on a liquid crystal display screen.
The characteristics of pressure-sensitive adhesive for polarizing film comprise the peeling force between adhesive and substrate, the peeling force between adhesive and release film, and adhesive durability. In general, after coating on a substrate, the pressure-sensitive adhesive is subjected to mature treatment at a specific temperature and relative humidity, thereby completing the cross-link reaction of the polymerizable components thereof. The peeling force between the adhesive and the substrate (called adhesive strength) is the most important factor.
The adhesive strength is measured through a method conforming to EIAJ ED-2521A of the Standard of Electric Industrial Association of Japan and indicated by g/25 mm. In general, the peeling force between adhesive of the polarizing film and substrate must be in a range between 100 to 1000 g/25 mm.
The adhesive strength of pressure-sensitive adhesive depends on the degree of crosslinking in the pressure-sensitive adhesive polymerized by a hardener. The polarizing film mounted on the substrate by the pressure-sensitive adhesive is apt to be easily peeled off or curled up when the pressure-sensitive adhesive has a lower degree of crosslinking, resulting in reduced yield and stability of LCD products.
A gel fraction is one index for measuring the degree of crosslinking of the polymerizable components of an adhesive. This index is obtained in such a manner that the adhesive was coated on a substrate and subjected to a mature treatment after several days. The solidified adhesive is scraped off the substrate, collected and weighted as W1. Next, the solidified adhesive is dissolved in ethyl acetate with ultrasonic agitation for 1 hr. The mixture is filtered, and the residual is collected, dried, and weighted as W2. The equation of gel fraction is:Gel fraction=(W2/W1)×100%
The degree of crosslinking of pressure sensitive adhesive used in an LCD can be estimated by gel fraction. The aforementioned method for measuring gel fraction, which is a destructive test, however, has complicated steps and long periods of time (5˜6 hr). Therefore, methods for measuring the degree of crosslinking of pressure sensitive adhesive with easy process and high convenience are desirable.